There is currently a rising demand for a variety of valuable metals which are used in a wide range of applications throughout the industrial arts and in scientific research. For example, neodymium, a rare-earth group metal, is finding increasing demand in applications that require permanent magnets. Zirconium, which has excellent corrosion resistance and low neutron absorption, has been employed extensively for structural purposes in the nuclear reactor industry and in chemical processing equipment. Titanium is a lightweight, noncorrosive, high strength metal that has found extensive use in the aircraft industry, the chemical processing industry and other energy-related fields.
At present, however, efforts to obtain large quantities of these valuable metals , particularly those in Groups IIIB and IVB or other metals that become extremely reactive at temperatures above about 500.degree. C., have suffered from various drawbacks. For example, neodymium is currently produced by a calciothermic reaction of neodymium trifluoride in a batch process where the temperature must be carefully controlled in order to reduce tantalum solubility in the neodymium. In this process, however, even with careful control there is generally some tantalum contamination present in the resulting neodymium metal product. Other methods of neodymium production have also failed to produce satisfactory levels of high-purity neodymium metal.
With regard to zirconium and titanium, these metals are currently produced by a magnesiothermic reduction commonly referred to as the Kroll process, such as described in U.S. Pat. No. 2,205,854. The Kroll process involves a batch procedure wherein the metal salt is fed into a reactor in the presence of molten magnesium under an inert atmosphere. This process requires several batch operational steps which are technically and economically disadvantageous to the production of the metal product, and the overall procedure suffers from the fact that it is a non-continuous operation that entails complex processing steps and high energy consumption.
It has been a particular problem with many transition group metals, such as those described above, that they are somewhat reactive at temperatures above about 500.degree. C. and as a result, when molten, have a strong tendency to react with most materials commonly used for their containment. This usually results in impurities that remain in the finished metal product. It is thus preferred that a reactor useful in purifying metals such as zirconium, neodymium and titanium be one wherein the enclosure will be nonreactive with the treated metals in the molten state. It will also be highly preferable that such a reactor provides for a reaction volume with sufficient residence time to complete the reaction, an input of heat sufficient to maintain the metals and other reactants in molten condition, and a system for mixing reactants and products to ensure reactant availability for reaction and product homogeneity, all in a continuous process wherein products and by-products are removed.
One such reactor which will be suitable for the preferred steps described above is disclosed in U.S. Pat. No. 3,775,091, incorporated herein by reference. The apparatus described therein is designed to melt refractory metals such as titanium, zirconium and their alloys in an induction-heated, liquid-cooled, segmented copper crucible. The bottom of the crucible is formed by the cooled melt material, and a continuous metal ingot of the desired material may be produced and withdrawn from the apparatus. The system is designed so that calcium fluoride and the refractory metal are fed into the crucible, and the calcium fluoride forms an insulating layer to protect the cooled copper. Water-cooled copper coils are also provided around the crucible in order to carry the alternating current for the induction heating.
It is thus a highly desirable object to develop a system whereby the apparatus as disclosed in U.S. Pat. No. 3,775,091 can be used to carry out the purification of a wide variety of metals in an efficient and continuous process, and it is also highly desirable to provide a method wherein a variety of metals can be recovered in purified form from a reactor that does not become reactive with the purified metal, that provides sufficient heat input and residence time to complete the reaction, and which ensures efficient reactant availability and product homogeneity.